With an inexpensive FM scanner and a pc anyone can receive and decode images from the National Oceanic and Atmospheric Administration Polar Orbiting Environmental Satellites.
These satellites offer the advantage of daily global coverage, by making nearly polar orbits roughly 14.1 times daily. Currently in orbit there is a morning and afternoon satellite, which provide global coverage four times daily.
The data we want to receive from these satellites is encoded in an audio signal called an Automatic Picture Transmission (APT). The broadcast transmission is composed of two image channels, telemetry information, and synchronization data.. All this data is transmitted as a horizontal scan line. A complete line is 2080 pixels long, with each image using 909 pixels and the remainder going to the telemetry and synchronization. Lines are transmitted at 2 per second, which equates to a 4160 words per second, or 4160 baud.
On NOAA POES system satellites, the two images are 4 km / pixel smoothed 8-bit images derived from two channels of the Advanced Very High Resolution Radiometer (AVHRR) sensor. Of the two images, one is typically long-wave infrared (10.8 micrometers) with the second switching between near-visible (0.86 micrometers) and mid-wave infrared (3.75 micrometers) depending on whether the ground is illuminated by sunlight.
Included in the transmission are a series of synchronization pulses, minute markers, and telemetry information.
The synchronization information, transmitted at the start of each video channel, allows the receiving software to align its sampling with the baud rate of the signal, which can vary slightly over time. The minute markers are four lines of alternating black then white lines which repeat every 60 seconds (120 lines)
The signal itself is a 256-level amplitude modulated 2400Hz subcarrier, which is then frequency modulated onto the 137 MHz-band RF carrier. Maximum subcarrier modulation is 87% (±5%), and overall RF bandwidth is 34 kHz. On NOAA POES satellites, the signal is broadcast at approximately 40dBm (10 watts) effective radiated power
An APT signal is continuously broadcast, with reception beginning at the start of the next line when the receiver is within radio range. Images can be received in real-time by relatively unsophisticated, inexpensive receivers during the time the satellite is within radio range, which typically lasts 8 to 15 minutes.
Now that we have a basic understanding of the signal let’s get down to the actually process of receiving and decoding that signal.
The Equipment you will need:
- FM Scanner that can receive on the 137 MHz-band.
- PC with WXtoImg software
- Cables to connect scanner to pc.
There are several software products to decode the ATP signal; the one we will be discussing is WXtoImg. This software comes in a freeware version that is very feature rich and will do everything you will need to receive and decode the images from the satellites. You can download WXtoImg from http://www.wxtoimg.com/ . Once you have it installed and your scanner connected to your sound card there are a few steps to setting up the software. First you will configure your receiving location by longitude and latitude so that the software can track satellite passes. Second you will click on help and read the entry on required calibrations. This will teach you how to set your volume levels and other calibrations that will need to be made on your first satellite pass.
Once you have the software setup you will need to program your scan with the frequencies used by the satellites. These frequencies are 137.6200, 137.5000, 137.9125, 137.1000.
Now that you are ready to receive the signal you can tell WXtoImg to wait for the next pass and auto decode. This is started simply by clicking on the file menu and then clicking on Record, the record dialog box will appear and you will click “Auto Record”. WXtoImg will now be in standby mode waiting for the next satellite to appear on the horizon. The bottom status bar will indicate when the next pass is with info about the satellite and what frequency it will be on. You can set your scanner to scan those frequencies and walk away or you can just set the frequency for the next pass.
When the satellite comes up on the horizon you should be able to hear the easy to identify sound of the signal. Once WXtoImg starts recording the signal you will see the image begain to apeair one line at a time. This will last from 5 -15 minutes depending on the elevation of the satellite pass. Once the pass is complete WXtoImg will attempt to decode the image and create false color and enhanced images from the original signal. WXtoImg will also add map overlays and an “X” marks the spot for your location.
Now that you are able to receive and decode APT signals from Weather Satellites let’s discuss what can be done to improve reception of the signal. Now you can receive and decode signals with a simple vertical antenna but there will be dips in the signal strength as the satellite passes over. Plus the antenna that is uses on the transmitter is right-hand circularly polarized so for best reception you will need an antenna that is right-hand circularly polarized and will not have many dip-outs as the antenna passes from horizon to horizon. A popular antenna for this uses is the Eggbeater design and it will work with a right or left hand circularly polarized signal and is great for receiving overhead signals. You can also use the crossed dipole and the quadrifilar helix antenna (QHA).
Also this will work with just about any receiver capable of reception in the 137-138MHz FM band but for best results requires a scanner with 30kHz – 50kHz bandwidth. It is not advisable to use just any scanner (most of which only support 15kHz or 230kHz bandwidths) if you intend to use it for weather satellite reception as the image quality will suffer, but you will still produce a picture.
There are many resources online that you can review for information about the status of the satellites in operation. http://www.oso.noaa.gov/poesstatus/ list the current satellites and their operation details.
Now you can operate your own weather satellite receiving station and get live images of the earth from space without the need of the internet or television.
Click here for a PDF doc with information on building an APT ground station in an article from GEO Quarterly No 1 by Less Hamliton.